In modern day telephone communication systems, digital data is employed for transmission via a switching matrix to various subscribers associated with the system. The use of digital data enables efficient transmission and enables the various system modules to be implemented by integrated circuit configurations, which are particularly well suited for the fabrication of digital circuitry.
The function of a D/A converter is to operate on a digital word or number and convert it to an analog voltage or current proportional to or indicative of the digital word. In a typical telephone system, subscribers communicate via the transmission of analog signals. In the digital telephone system, the analog signals are converted into digital signals. Hence, prior to application of a received digital signal to a subscriber, one converts the signal back to an analog signal and hence employs a digital to analog converter. A D/A converter should preferably provide a continuous analog output, which is manifested by a smooth curve drawn through the sample points.
Many techniques exist in the prior art for the conversion of digital signals to analog signals and many such techniques and apparatus are well known. Essentially, the prior art employed ladder networks which are switched to hold each sample constant for one period. This technique suppresses inverted components in the output by use of a low pass filter. However, the ladder circuitry is extremely expensive as it requires high tolerance, tracked components and as such are not compatible with integrated circuit technology.
Hence, various alternatives have been proposed such as digital techniques employing a rate multiplier. In such schemes, a rate multiplier is used to produce an output pulse stream having a mean density proportional to a clock frequency times the input digital number. The input number is changing at each sample instant and hence, the clock frequency must be equal to the sampling frequency times the number of possible levels in the input number. For example, a 12 bit linear PCM signal at an 8KHz sampling rate would require a clock frequency of about 32MHz (megahertz). This rate is extremely high and hence, to compromise, one converts the PCM signal to sign, magnitude and scaling components. The magnitude is applied to a rate multiplier operating at a lower clock frequency, the output of which is scaled and signed by analog means.
Another useful technique is described in U.S. Pat. No. 4,109,110 entitled DIGITAL TO ANALOG CONVERTER issued on Aug. 22, 1978 to M. J. Gingell and assigned to the International Standard Electric Corporation and employs digital means for increasing the sampling rate of an input digital signal. The increased rate signal is of a lower number of bits per sample to enable final digital to analog conversion to be facilitated by a relatively simple digital to analog converter in conjunction with a rate multiplier, to provide an output pulse strtess having a mean density proportional to the analog signal amplitude. This output stream is passed through a low pass filter to yield the analog signal. The system described in this patent is applicable for use in telephony systems using PCM signals.
Such converters and others operating at an 8KHz sampling rate or frequency require highly accurate specifications for the analog filters as well as in the case of the ladder arrangements, a large number of bits per sample. In a telephone system, these factors add additional expense to the line circuit.
Thus, the technique described in the above patent uses a digital interpolator to raise or increase the word rate to 256 KHz to 12 bits and then to reduce the word length to 4 bits. The rate multiplier operates with the 256 KHz signal at 4 bits to implement a pulse density modulator operating at a clock rate of 4,096 KHz and is followed by a low pass filter.
It would be desirable to eliminate the rate multiplier in a digital to analog conversion scheme and to provide a pulse stream which can be directly applied to a low pass filter to retrieve an accurate representation of the analog signal.
An object of the present invention is to provide an improved D/A converter particularly adapted for use in a digital telephone line circuit and capable of being implemented with digital integrated circuitry. The digital to analog converter to be described is therefore compatible with integrated circuit techniques and hence, enables digital to analog conversion in economical and reliable configurations.